/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/MeshOpPreviewHelpers.h
If set to true, then it will be assumed that the mesh topology (i.e. triangle/edge connectivity) remains constant, which will allow updates after the first one to modify existing render proxy buffers rather than creating entirely new ones. This will give a significant speedup to tools that do not add/remove vertices or affect their connectivity.
void SetIsMeshTopologyConstant ( bool bOn, EMeshRenderAttributeFlags ChangingAttributes )